The present invention relates to a method for the preparation of an alkoxy-containing organopolysiloxane or, more particularly, to a method for the preparation of a diorganopolysiloxane terminated at both molecular chain ends each with a monohydrocarbyl dialkoxy silyl group which is useful as a principal ingredient in a room temperature-curable organopolysiloxane composition.
As is well known, the principal ingredient in a class of room temperature-curable organopolysiloxane compositions is a diorganopolysiloxane terminated at both molecular chain ends each with a silanolic hydroxy group, i.e. a hydroxy group directly bonded to the silicon atom, and the curing reaction of the composition into a cured rubber proceeds by the dealcoholation condensation between the above mentioned silanol groups and the alkoxy groups of the alkoxy-containing organosilane or a partial hydrolysis-condensation product thereof to act as a crosslinking agent formulated in the composition as another essential ingredient. Because the condensation product formed by the condensation reaction is an alcoholic compound having no corrosiveness and irritative odor, room temperature-curable organopolysiloxane compositions of this type are advantageously used as an adhesive of various electronic materials. These compositions are disadvantageous, however, in respect of their relatively low velocity of curing and poor storability even under an anhydrous condition to lose curability. The instability of the composition in storage is presumably due to the hydrolytic decomposition of the alkoxy groups of the crosslinking agent by the reaction with the silanol groups. It is known that a solution of this problem can be obtained by replacing the silanol-terminated diorganopolysiloxane with a diorganopolysiloxane having an alkoxy group bonded to each of the terminal silicon atoms and a room temperature-curable organopolysiloxane composition of dealcoholation type formulated with such an alkoxyterminated diorganopolysiloxane has excellent storability and exhibits good curing behavior.
The above mentioned alkoxy-containing organopolysiloxane is conventionally prepared by the condensation reaction of a silanol-terminated diorganopolysiloxane and an organosilane compound having alkoxy groups and a hydrolyzable group and represented by the general formula EQU (R.sup.6 O).sub.2 (R.sup.3)SiY,
in which R.sup.3 is a monovalent group selected from the class consisting of methyl, ethyl, propyl, phenyl and 3,3,3-trifluoropropyl groups, R.sub.6 is a halogen-substituted or unsubstituted monovalent hydrocarbon group having 1 to 8 carbon atoms and Y is a hydrolyzable atom or group excepting alkoxy groups. In view of the reactivity with the silanol groups in the diorganopolysiloxane, the hydrolyzable group denoted by Y is preferably a halogen atom since a considerable amount of the silanol groups may remain unreacted when the hydrolyzable group denoted by Y is not a halogen atom due to the low reactivity. The dehydrohalogenation condensation between such a halogenosilane and the silanol-terminated diorganopolysiloxane involves several industrially difficult problems including the use of a relatively expensive acid acceptor such as triethyl amine, pyridine, picoline and the like in the reaction mixture and disposal of the hydrohalide of the acid acceptor as a by-product of the reaction.